Rhododendron is a genus of shrubs and small to (rarely) large trees, the smallest species growing to 10–100 cm (4–40 in) tall, and the largest, R. protistum var. giganteum, reported to 30 m (100 ft) tall.[6][7] The leaves are spirally arranged; leaf size can range from 1–2 cm (0.4–0.8 in) to over 50 cm (20 in), exceptionally 100 cm (40 in) in R. sinogrande. They may be either evergreen or deciduous. In some species, the undersides of the leaves are covered with scales (lepidote) or hairs (indumentum). Some of the best known species are noted for their many clusters of large flowers. There are alpine species with small flowers and small leaves, and tropical species such as sectionVireya that often grow as epiphytes. Species in this genus may be part of the heath complex in oak-heath forests in eastern North America.[8][9]

They have frequently been divided based on the presence or absence of scales on the abaxial (lower) leaf surface (lepidote or elepidote). These scales, unique to subgenus Rhododendron, are modified hairs consisting of a polygonal scale attached by a stalk.[2]

Rhododendron is the largest genus in the family Ericaceae, with as many as 1,024 species,[11] (though estimates vary from 850–1000[12]) and is morphologically diverse. Consequently, the taxonomy has been historically complex.[10]

Linnaeus' six species of Azalea were Azalea indica, A. pontica, A. lutea, A. viscosa, A. lapponica and A. procumbens (now Kalmia procumbens), which he distinguished from Rhododendron by having five stamens, as opposed to ten. As new species of what are now considered Rhododendron were discovered, if they seemed to differ significantly from the type species they were assigned to separate genera. For instance Rhodora (Linnaeus 1763) for Rhododendron canadense, Vireya (Blume 1826)[17] and Hymenanthes (Blume 1826) for Rhododendron metternichii, now R. degronianum. Meanwhile, other botanists such as Salisbury (1796)[18] and Tate (1831)[19] began to question the distinction between Azalea and Rhododendron, and finally in 1836, Azalea was incorporated into Rhododendron[20] and the genus divided into eight sections. Of these Tsutsutsi (Tsutsusi), Pentanthera, Pogonanthum, Ponticum and Rhodora are still used, the other sections being Lepipherum, Booram, and Chamaecistus. This structure largely survived till recently (2004), following which the development of molecular phylogeny led to major re-examinations of traditional morphological classifications,[15][16] although other authors such as Candolle, who described six sections,[21] used slightly different numeration.

Soon, as more species became available in the nineteenth century so did a better understanding of the characteristics necessary for the major divisions. Chief amongst these were Maximovicz's Rhododendreae Asiae Orientali[22] and Planchon. Maximovicz used flower bud position and its relationship with leaf buds to create eight "Sections".[23]Bentham and Hooker used a similar scheme, but called the divisions "Series".[24] It was not until 1893 that Koehne appreciated the significance of scaling and hence the separation of lepidote and elepidote species. The large number of species that were available by the early twentieth century prompted a new approach when Balfour introduced the concept of grouping species into series. The Species of Rhododendron[25] referred to this series concept as the Balfourian system. That system continued up to modern times in Davidian's four volume The Rhododendron Species.[26]

The next major attempt at classification was by Sleumer who from 1934 began incorporating the Balfourian series into the older hierarchical structure of subgenera and sections, according to the International Code of Botanical Nomenclature, culminating in 1949 with his "Ein System der Gattung Rhododendron L.",[27] and subsequent refinements.[28][29] Most of the Balfourian series are represented by Sleumer as subsections, though some appear as sections or even subgenera. Sleumer based his system on the relationship of the flower buds to the leaf buds, habitat, flower structure, and whether the leaves were lepidote or non-lepidote. While Sleumer's work was widely accepted, many in the United States and the United Kingdom continued to use the simpler Balfourian system of the Edinburgh group.

Sleumer's system underwent many revisions by others, predominantly the Edinburgh group in their continuing Royal Botanic Garden Edinburgh notes. Cullen of the Edinburgh group, placing more emphasis on the lepidote characteristics of the leaves, united all of the lepidote species into subgenus Rhododendron, including four of Sleumer's subgenera (Rhododendron, Pseudoazalea, Pseudorhodorastrum, Rhodorastrum).[29][30] In 1986 Philipson & Philipson raised two sections of subgenus Aleastrum (Mumeazalea, Candidastrum) to subgenera, while reducing genus Therorhodion to a subgenus of Rhododendron.[31] In 1987 Spethmann, adding phytochemical features proposed a system with fifteen subgenera grouped into three 'chorus' subgenera.[32]

A number of closely related genera had been included together with Rhododendron in a former tribe, Rhodoreae. These have been progressively incorporated into Rhododendron.[33] Chamberlain and Rae moved the monotypic section Tsusiopsis together with the monotypic genus Tsusiophyllum into section Tsutsusi,[34] while Kron & Judd reduced genus Ledum to a subsection of section Rhododendron.[35] Then Judd & Kron moved two species (Rhododendron schlippenbachii, R. quinquefolium) from section Brachybachii, subgenus Tsutsusi and two from section Rhodora, subgenus Pentanthera (R. albrechtii, R. pentaphyllum) into section Sciadorhodion, subgenus Pentanthera.[36] Finally Chamberlain brought the various systems together in 1996, with 1,025 species divided into eight subgenera. For a comparison of the Sleumer and Chamberlain schemata see Table 1 of Goetsch (2005).[2][10][16][37][38]

The era of molecular analysis rather than descriptive features can be dated to the work of Kurashige (1988) and Kron (1997) who used matKsequencing. Later Gao et al. (2002) used ITS sequences[39] to determine a cladistic analysis. They confirmed that the genus Rhododendron was monophyletic, with subgenus Therorhodion in the basal position, consistent with the matK studies. Following publication of the studies of Goetsch et al. (2005) with RPB2,[2] there began an ongoing realignment of species and groups within the genus, based on evolutionary relationships. Their work was more supportive of Sleumer's original system than the later modifications introduced by Chamberlain et al..[2][40]

The major finding of Goetsch and colleagues was that all species examined (except R. camtschaticum, subgenus Therorhodion) formed three major clades which they labelled A, B and C, with the subgenera Rhododendron and Hymenanthes nested within clades A and B as monophyletic groups respectively. By contrast subgenera Azaleastrum and Pentanthera were polyphyletic, while R. camtschaticum appeared as a sister to all other rhododendrons. The small polyphyletic subgenera Pentanthera and Azaleastrum were divided between two clades. The four sections of Pentanthera between clades B and C, with two each, while Azaleastrum had one section in each of A and C.

Thus subgenera Azaleastrum and Pentanthera needed to be dissassembled, and Rhododendron, Hymenanthes and Tsutsusi correspondingly expanded. In addition to the two separate genera included under Rhododendron by Chamberlain (Ledum, Tsusiophyllum), Goetsch et al.. added Menziesia (Clade C). Despite a degree of paraphyly, the subgenus Rhododendron was otherwise untouched with regard to its three sections but four other subgenera were eliminated and one new subgenus created, leaving a total of five subgenera in all, from eight in Chamberlain's scheme. The discontinued subgenera are Pentanthera, Tsutsusi, Candidastrum and Mumeazalea, while a new subgenus was created by elevating subgenus Azaleastrum section Choniastrum to subgenus rank.

Subgenus Pentanthera (deciduous azaleas) with its four sections was dismembered by eliminating two sections and redistributing the other two between the existing subgenera in clades B (Hymenanthes) and C (Azaleastrum), although the name was retained in section Pentanthera (14 species) which was moved to subgenus Hymenanthes. Of the remaining three sections, monotypic Viscidula was discontinued by moving Rhododendron nipponicum to Tsutsusi (C), while Rhodora (2 species) was itself polyphyletic and was broken up by moving Rhododendron canadense to section Pentanthera (B) and Rhododendron vaseyi to section Sciadorhodion, which then became a new section of subgenus Azaleastrum (C).

Subgenus Tsutsusi (C) was reduced to section status retaining the name, and included in subgenus Azaleastrum. Of the three minor subgenera, all in C, two were discontinued. The single species of monotypic subgenus Candidastrum (Rhododendron albiflorum) was moved to subgenus Azaleastrum, section Sciadorhodion. Similarly the single species in monotypic subgenus Mumeazalea(Rhododendron semibarbatum) was placed in the new section Tsutsusi, subgenus Azaleastrum. Genus Menziesa (9 species) was also added to section Sciadorhodion. The remaining small subgenus Therorhodion with its two species was left intact. Thus two subgenera, Hymenanthes and Azaleastrum were expanded at the expense of four subgenera that were eliminated, although Azaleastrum lost one section (Choniastrum) as a new subgenus, since it was a distinct subclade in A. In all, Hymenanthes increased from one to two sections, while Azaleastrum, by losing one section and gaining two increased from two to three sections.[2] (See schemata under Subgenera) (Table 1.)[40]

Table 1.: Taxonomic changes within genus Rhododendron

Chamberlain (1996)

Goetsch (2005)

Genus

Subgenus

Section

Species

Subgenus

Section

Menziesa

9 species

Azaleastrum

Sciadorhodion

Rhododendron

Candidastrum

R. albiflorum

Pentanthera

Sciadorhodion

4 species

Rhodora

R. vaseyi

R. canadense

Hymenanthes

Pentanthera

Pentanthera

14 species

Viscidula

R. nipponicum

Azaleastrum

Tsutsusi

Tsutsusi

Brachycalyx

15 species

Tsutsusi

65 species

Mumeazalea

R. semibarbatum

Azaleastrum

Choniastrum

11 species

Choniastrum

Subsequent research has supported the revision by Goetsch, although has largely concentrated on further defining the phylogeny within the subdivisions.[41] In 2011 the two species of Diplarche were also added to Rhododendron, incertae sedis.[42]

Terminology from the Sleumer (1949) system is frequently found in older literature, with five subgenera and is as follows;

Subgenus Lepidorrhodium Koehne: Lepidotes. 3 sections

Subgenus Eurhododendron Maxim.: Elipidotes.

Subgenus Pseudanthodendron Sleumer: Deciduous azaleas. 3 sections

Subgenus Anthodendron Rehder & Wilson: Evergreen azaleas. 3 sections

Subgenus Azaleastrum Planch.: 4 sections

In the later traditional classification, attributed to Chamberlain (1996), and as used by horticulturalists and the American Rhododendron Society,[43]Rhododendron has eight subgenera based on morphology, namely the presence of scales (lepidote), deciduousness of leaves, and the floral and vegetative branching patterns, after Sleumer (1980).[2][10][29] These consist of four large and four small subgenera. The first two subgenera (Rhododendron and Hymenanthes) represent the species commonly considered as 'Rhododendrons'. The next two smaller subgenera (Pentanthera and Tsutsusi) represent the 'Azaleas'. The remaining four subgenera contain very few species.[44] The largest of these is subgenus Rhododendron, containing nearly half of all known species and all of the lepidote species.

For a comparison of the Sleumer and Chamberlain systems, see Goetsch et al. (2005) Table 1.[2]

This division was based on a number of what were thought to be key morphological characteristics. These included the position of the inflorescence buds (terminal or lateral), whether lepidote or elepidote, deciduousness of leaves, and whether new foliage was derived from axils from previous year's shoots or the lowest scaly leaves (Table 2.).

The larger subgenera are further subdivided into sections and subsections[44] Some subgenera contain only a single section, and some sections only a single subsection. Shown here is the traditional classification, with species number after Chamberlain (1996), but this scheme is undergoing constant revision. Revisions by Goetsch et al. (2005)[2] and by Craven et al. (2008)[41] shown in (parenthetical italics). Older ranks such as Series (groups of species) are no longer used but may be found in the literature, but the American Rhododendron Society still uses a similar device, called Alliances[43]

The 300 tropical species within the Vireya section of subgenus Rhododendron occupy the Malay archipelago from their presumed Southeast Asian origin to Northern Australia, with 55 known species in Borneo and 164 in New Guinea. The species in New Guinea are native to subalpine moist grasslands at around 3,000 metres above sea level in the Central Highlands.[45] Subgenera Rhododendron and Hymenanthes, together with section Pentanthera of subgenus Pentanthera are also represented to a lesser degree in the Mountainous areas of North America and Western Eurasia. Subgenus Tsutsusi is found in the maritime regions of East Asia (Japan, Korea, Taiwan, East China), but not in North America or Eurasia.[2][23]

Rhododendron ponticum has become invasive in Ireland[46] and the United Kingdom.[47] It is an introduced species, spreading in woodland areas and replacing the natural understory. R. ponticum is difficult to eradicate, as its roots can make new shoots.

A number of insects either target rhododendrons or will opportunistically attack them. Rhododendron borers and various weevils are major pests of rhododendrons, and many caterpillars will preferentially devour them.

Major diseases include Phytophthora root rot, stem and twig fungal dieback; Ohio State University Extension provides information on maintaining health of rhododendrons.[48]

Rhododendron bud blast, a fungal condition that causes buds to turn brown and dry, and not open, is caused by the fungus Pycnostysanus azaleae, which may be brought to the plant by the rhododendron leafhopper, Graphocephala fennahi.[49]:562

In addition, rhododendrons can easily be suffocated by other plants or evergreen trees that grow up around them and block sunlight.

Rhododendrons can be propagated by air layering or stem cuttings.[49]:540–541 They can self-propagate by sending up shoots from the roots. Sometimes an attached branch that has drooped to the ground will root in damp mulch, and the resulting rooted plant then can be cut off the parent rhododendron.

Rhododendrons are often valued in landscaping for their structure, size, flowers, and the fact that many of them are evergreen.[50] Azaleas are frequently used around foundations and occasionally as hedges, and many larger-leafed rhododendrons lend themselves well to more informal plantings and woodland gardens, or as specimen plants. In some areas, larger rhododendrons can be pruned to encourage more tree-like form, with some species such as Rhododendron arboreum and R. falconeri eventually growing to 10–15 m or more tall.[50]

Rhododendrons are grown commercially in many areas for sale, and are occasionally collected in the wild, a practice now rare in most areas. Larger commercial growers often ship long distances; in the United States, most of them are on the west coast (Oregon, Washington state and California). Large-scale commercial growing often selects for different characteristics than hobbyist growers might want, such as resistance to root rot when overwatered, ability to be forced into budding early, ease of rooting or other propagation, and saleability.[51]

In the Indian state of Himachal Pradesh, rhododendron flowers have been used for some time to make popular fruit and flower wines. The industry is promoted by the state government with tax benefits, looking to promote this industry as a full-fledged subclass of its economy.[52]

Like other ericaceous plants, most rhododendrons prefer acid soils with a pH of roughly 4.5–5.5; some tropical Vireyas and a few other rhododendron species grow as epiphytes and require a planting mix similar to orchids. Rhododendrons have fibrous roots and prefer well-drained soils high in organic material. In areas with poorly drained or alkaline soils, rhododendrons are often grown in raised beds using media such as composted pine bark.[63] Mulching and careful watering are important, especially before the plant is established.

A new calcium-tolerant stock of rhododendrons (trademarked as 'Inkarho') has been exhibited at the RHSChelsea Flower Show in London (2011). Individual hybrids of rhododendrons have been grafted on to a rootstock on a single rhododendron plant that was found growing in a chalk quarry. The rootstock is able to grow in calcium-rich soil up to a pH of 7.5.[64][65]

Rhododendron species have long been used in traditional medicine.[69][70] Animal studies and in vitro research has identified possible anti-inflammatory and hepatoprotective activities which may be due to the antioxidant effects of flavonoids or other phenolic compounds and saponins the plant contains.[71] Xiong et al. have found that the root of the plant is able to reduce the activity of NF-κB in rats.[72]

Some species of rhododendron are poisonous to grazing animals because of a toxin called grayanotoxin in their pollen and nectar. People have been known to become ill from eating honey made by bees feeding on rhododendron and azalea flowers. Xenophon described the odd behaviour of Greek soldiers after having consumed honey in a village surrounded by Rhododendron ponticum during the march of the Ten Thousand in 401 BC. Pompey's soldiers reportedly suffered lethal casualties following the consumption of honey made from Rhododendron deliberately left behind by Pontic forces in 67 BC during the Third Mithridatic War. Later, it was recognized that honey resulting from these plants has a slightly hallucinogenic and laxative effect.[73] The suspect rhododendrons are Rhododendron ponticum and Rhododendron luteum (formerly Azalea pontica), both found in northern Asia Minor. Eleven similar cases have been documented in Istanbul, Turkey during the 1980s.[74] Rhododendron is extremely toxic to horses, with some animals dying within a few hours of ingesting the plant, although most horses tend to avoid it if they have access to good forage. The effects of R. ponticum was mentioned in the 2009 film Sherlock Holmes as a proposed way to arrange a fake execution.[75] It was also mentioned in the third episode of Season 2 of BBC's Sherlock, speculated to have been a part of Sherlock's fake death scheme.

In Joyce'sUlysses, rhododendrons play an important role in Leopold and Molly's early courtship: Molly remembers them in her soliloquy – "the sun shines for you he said the day we were lying among the rhododendrons on Howth head in the grey tweed suit and his straw hat the day I got him to propose to me". Jasper Fforde a British author, also uses rhododendron as a motif throughout many of his published books. See Thursday Next series,[77] and Shades of Grey.[78]
Amongst the Zomi tribes in India and Myanmar, "Rhododendrons" called "Ngeisok" is used in a poetic manner to signify a lady.[citation needed]

In Daphne Du Maurier's novel Rebecca, the character of Rebecca is associated with "blood red" rhododendrons throughout the novel, perhaps due to the toxic roots of the plant mirroring the poisonous character of Rebecca. On the other hand, azaleas (a type of rhododendron) represent the second Mrs. De Winter.[citation needed]

The rhododendron is the national flower of Nepal, where the flower is considered edible and enjoyed for its sour taste. The pickled flower can last for months and the flower juice is also marketed.[79]:51 The flower, fresh or dried, is added to fish curry in the belief that it will soften the bones.[79]:53 The juice of rhododendron flower is used to make a squash called burans (named after the flower) in the hilly regions of Uttarakhand. It is admired for its distinctive flavor and color.[80]

In the UK the forerunner of the Rhododendron, Camellia and Magnolia Group (RCMG), The Rhododendron Society was founded in 1916.[81] while in Scotland species are being conserved by the Rhododendron Species Conservation Group.[82]

^Schafale, M. P. and A. S. Weakley. 1990. Classification of the natural communities of North Carolina: third approximation. North Carolina Natural Heritage Program, North Carolina Division of Parks and Recreation.

^ abcSleumer, H (1980). "Past and present taxonomic systems of Rhododendron based on macromorphological characters". In Luteyn, J.L. & O'Brien, M.E. (eds.). Contributions Toward a Classification of Rhododendron: Proceedings of the International Rhododendron Conference. International Rhododendron Conference (The New York Botanical Garden, 15–17 May 1978). New York: New York Botanical Garden Press. pp. 19–26. ISBN978-0-89327-221-0.